The subject matter disclosed herein relates generally to magnetic resonance imaging systems, and more particularly to systems and methods for disabling coils in magnetic resonance imaging systems.
Magnetic Resonance Imaging (MRI) systems include a magnet, such as a superconducting magnet that generates a temporally constant (i.e., uniform and static) primary or main magnetic field. MRI data acquisition is accomplished by exciting magnetic moments within the primary magnetic field using magnetic gradient coils. For example, in order to image a region of interest, the magnetic gradient coils are energized to impose a magnetic gradient to the primary magnetic field. Transmit radio-frequency (RF) coils are then pulsed to create RF magnetic field pulses in a bore of an MRI scanner to selectively excite a volume corresponding to the region of interest in order to acquire MR images of the region of interest using receive RF coils. During the transmission of the RF magnetic field pulses, the receive RF coils are decoupled or disabled and during reception the transmit RF coils are decoupled or disabled. The resultant image that is generated shows the structure and function of the region of interest.
In conventional MRI systems, isolating the receive RF coils from the transmit field is provided using active and passive disabling or decoupling networks. For example, in birdcage type RF coils a large number of disabling circuits are needed to provide decoupling, with each typically having one or more high power diodes, for example, PIN diodes. These diodes are not only expensive, but easy to break. Depending on the placement of the diodes, detuning capacitors may be used, which degrades the uniformity of the birdcage coil. Additionally, more heating may result from large RF currents, which reduces reliability. Moreover, in multi-nuclear spectroscopy imaging wherein multiple coils are tuned to different resonant frequencies, additional disabling circuits are needed for each of the coils. In some of these multi-nuclear spectroscopy imaging systems the capacitive distribution is changed to provide disabling, but requires a system design change.